Palladium nanoparticles in Suzuki cross-couplings: Tapping into the potential of tris-imidazolium salts for nanoparticle stabilization

Inspired by the proclivity of various palladium sources to form nanoparticles in imidazolium-based ionic liquids, we now report that tris-imidazolium salts bearing hexadecyl chains and a bridging mesitylene moiety are potent stabilizers of palladium nanoparticles efficiently prepared via a Chaudret-type hydrogenation of the bis(dibenzylideneacetone)palladium(0). The palladium nanoparticles have been isolated in pure form and characterized by 1H nuclear magnetic resonance, transmission electron microscopy, electron diffraction and dynamic light scattering. The new materials proved effective in Suzuki cross-coupling at a loading of 0.2% palladium. Thus, using a tris-imidazolium iodide-palladium material, a series of biaryl products has been prepared starting from aryl bromides and some activated chlorides. The possibility that this catalytic activity might be due to the formation of palladium N-heterocyclic carbenes has been addressed through solid state 13C NMR and the synthesis of an imidazolium analogue in which the acidic 2-H was replaced with a methyl group. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.